Cerebral haemodynamicsThe cardiac output is about 5 l/min of blood for a resting adult. Blood flow to the brain isabout 14% of this, or 700 ml/min. For any part of the body, the blood flow can be calculatedusing the simple formulaResistancePressureBlood flow = (2.1)Pressure in the arteries is generated by the heart which pumps blood from its left ventricleinto the aorta. (Since pressure was historically measured with a mercury manometer, theunits are commonly expressed in terms of [mm Hg], although the official SI unit is thePascal [Pa].) Resistance arises from friction, and is proportional to the following expression(Vessel Diameter)4Vessel LengthResistance ∝Viscosity×(2.2)Hence blood flow is slowest in the small vessels of the capillary bed, thus allowing time forthe exchange of nutrients and oxygen to surrounding tissue by diffusion through thecapillary walls.Approximately 75% of total blood volume is ‘stored’ in the veins which, because oftheir high capacity, act as reservoirs. Their walls distend and contract in response to the
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